Cell Bio Notes PDF

Cell Biology Lesson & book notes By Carolina Pereira 2024/2025 Lesson 1: Cells & organelles, membrane structure Eukaryotic cell Components & functions of eukaryotic cell are in the next slide How do animal cells differ from plant cells? Plants have a cell wall a central vacuole and...

Cell Biology Lesson & book notes By Carolina Pereira 2024/2025 Lesson 1: Cells & organelles, membrane structure Eukaryotic cell Components & functions of eukaryotic cell are in the next slide How do animal cells differ from plant cells? Plants have a cell wall a central vacuole and chloroplast but no centrosomes Nucleus Eukaryotic Stores genetic information Nucleolus Eukaryotic Makes ribosomes Cytoplasm All cells Contains the contents of the cell Cytosol All cells Matrix that holds water & nutrients Cytoskeleton Eukaryotic Structure, support and transport Ribosome All Cells Makes protein Makes proteins for the Rough Endoplasmic Reticulum Eukaryotic endomembrane system Smooth Endoplasmic Reticulum Eukaryotic Detoxifies the cell and makes lipids Golgi Apparatus Eukaryotic Sorts and ships proteins Mitochondria Eukaryotic Makes energy Lysosome Eukaryotic, animal cells only Removes unwanted material & waste Regulate biochemical pathways that Peroxisome Eukaryotic involve oxidation Vacuoles Eukaryotic Store water and nutrients Vesicles Eukaryotic Transport materials around the cell A thin flexible barrier that separates Cell Membrane All the cell from its environment Cell Wall Plants, fungi and prokaryotes Rigid barrier that protects the cell  Endosymbiosis theory An early eukaryotic cell, already possessing An ancestral anaerobic predator cell (an archaeon) is mitochondria, engulfed a photosynthetic bacterium (a thought to have engulfed the bacterial ancestor of cyanobacterium) and retained it in symbiosis. Present- mitochondria, initiating a symbiotic relationship. Clear day chloroplasts are thought to trace their ancestry evidence of a dual bacterial and archaeal inheritance can be back to a single species of cyanobacterium that was discerned today in the genomes of all eukaryotes. adopted as an internal symbiont (an endosymbiont) over a billion years ago.  The plasma membrane Functions: Defines the cell Provides shape and strength (together with cytoskeleton) Forms a selective barrier Important in facilitating: Intercellular communication Properties Exchange of substances (import & Consists of lipids, proteins and sterols (Cholesterol) export) Forms a fluid lipid-bilayer Cell growth and motility Is held together via noncovalent interactions  Phospholipids Consist of: Polar head group containing a phosphate Two non-polar hydrocarbon tails (usually two fatty acids) They are amphiphilic= have both hydrophilic and hydrophobic parts Important classes of phospholipids: phosphoglycerides (derived from glycerol) & sphingolipids (derived from sphingosine) Sterols  Cholesterol Cholesterol stabilizes membrane fluidity: At high temperatures decreased fluidity At low temperature  increased fluidity Cholesterol immobilizes the upper part of the fatty acid chain. Makes the membrane less deformable and therefore less permeable. Sterols  Saturated vs unsaturated fatty acids Saturated fatty acids make the membrane less fluid Higher ratio means relatively more saturated fatty acids (in relation to the ratio in different animals in different temperatures in their environment.  Lipid rafts The cell membrane is not homogeneous Lipid rafts are dynamic structures enriched in cholesterol, sphingolipids, glycolipids & certain proteins The membrane is thicker in lipid rafts due to its composition Lipid rafts form platforms for protein interactions & signalling Transmembrane/integral protein Integrated in the entire membrane 1: single -helix 2: multiple -helix 3: -barrel Peripheral proteins Attached to the outside of the cell membrane 4: Attached via an α-helix 5 and 6: covalently bound to a lipid chain 7 and 8: Bound via other proteins  Membrane proteins Give functional properties to the cell membrane Can associate in various ways with the membrane Are amphiphilic Structure and properties Contain hydrophilic & hydrophobic amino acids Most transmembrane proteins cross the lipid bilayer with an α-helix (every peptide bond forms hydrogen bonds in an α-helix) No water present within the bilayer  all peptide bonds form hydrogen bonds with each other (most efficient in α-helix) Transmembrane part is 20-30 a.a long Transmembrane α-helices interact with each other for proper structure & function  Hydropathy plots predict membrane proteins Uses the amino acid composition to predict transmembrane regions Hydropathy index = The free energy (a positive value) needed to transfer the segments of a polypeptide chain from a nonpolar solvent to water is calculated from the amino acid composition of each segment The hydropathy index is plotted on the Y axis as a function of its location in the chain Peaks in the hydropathy index appear at the positions of hydrophobic segments in the amino acid sequence Membrane proteins are different at the cytosolic vs non-cytosolic side Most membrane proteins are glycosylated The sugar residues are added in the lumen of the ER and the Golgi apparatus; Therefore, these sugar residues are found on the extracellular side of the protein Disulfide (S–S) bonds form between cysteines on the non-cytosolic side of the membrane & Disulfide bonds cannot form at the cytosolic side due to the reducing environment. Carbohydrate layer on the surface Carbohydrates can be attached to proteins & lipids The glycocalyx is a term used to describe the carbohydrate rich zone on the cell surface. Functions of glycocalyx Protection to chemical & mechanical stress Prevent unwanted cell-cell interactions  Lesson 2: Membrane transport The cell membrane is selectively permeable Membrane permeability depends on polarity, charge & size How do solutes get across a membrane? Membrane transport proteins Multipass membrane Interact weaker with Bind more strongly to solute proteins solute Undergo sequential Are specific for one or a few Form narrow pores conformational changes molecules Can be open or closed Never open on both sides Faster transport Slower transport  Types of transporters Transporters can be divided in 3 types: Uniporters Symporters (coupled transport) Antiporters (coupled transport) How do solutes get across a Active transport – membrane? Why ? Passive transport: Transport of essential substances into the no energy required – along concentration cell, even though the concentration outside gradient the cell is lower. simple diffusion or via channels (all) or Waste substances must be removed from transporters (some) the cell, even if the concentration of such Active transport: substances outside the cell is higher. costs energy (ATP) – against concentration Allows the cell to maintain a constant and gradient unbalanced concentration of ions (K+, via transporters (some) Na+, Ca2+, H+).  Active transporters Three modes of active transport coupled transport (secondary active transport) ATP-driven (primary active transport) light-driven (primary active transport) The Na+/K+ ATPase pump It creates high Na+ outside the cell, high K+ inside the cell For every molecule of ATP hydrolysed, 3 Na+ are out, and 2 K+ are in Electrochemical gradient of Na+ can be used for transport Mechanism of glucose transport fuelled by a Na+ gradient in (kidney/intestine)  Transport of water Can water pass the cell membrane?  Osmotic pressure Sometimes water needs to be transported more rapidly, which is facilitated by aquaporins. Passive channel in kidney & glands Allow passage of water Block passage of ions HOW? Asparagine selectivity filter: Prevents passage of H+ Pore too small for hydrated ions Role of aquaporins in fluid secretion No way to actively transport water over the membrane. Ions (for example Na+ and Cl-) are actively transported into the lumen. Water follows the osmotic gradient  K+ leak channel & Na+/K+ pump Ion channels Function in transport of ions across the membrane Are selective for specific ions (which limits the rate of passage) 1 open and 2 closed conformations (closed and inactivated) Respond to electrical, mechanical and chemical signals Both are very important to maintain the resting membrane potential Na+/K+ pump creates high Na+ outside the cell, high K+ inside the cell  20% the membrane potential (& its maintenance) K+ leaks out of the cell until electrochemical gradient for K + is 0  Maintaining the membrane potential Na+ leaks out of the cell until electrochemical gradient for Na+ is 0  Transport channels Differences in gating of ion channels  Neurons & voltage-gated channels Plasma membrane of all excitable cells contain voltage-gated ion channels Responsible for action potentials  transport signals throughout body Depolarization: shift in membrane potential to a less negative value inside Important role voltage-gated Na+ & K+ channels Action potentials in neurons Resting potential = -70 mV Action potential threshold = -55 mV Action potential: 1. Voltage-gated Na+ channels open depolarization (+ 30 mV) 2. Voltage-gated K+ channels open  repolarization (